Omar Epps is an actor; he has a supporting role in the Fox TV series "House."
Chris Daughtry was on American Idol in 2006. He lost, but went on to have a very successful recording career--rock music.
Topher Grace is an actor; I only know of his from a movie he was in with Dennis Quaid. Though much younger, Grace becomes Quaid's boss and falls for Quaid's daughter. Sorry, I can't remember the name of the film.
No clue about Dax Shepard. The only Daxes I know of were on Star Trek: Deep Space Nine.

Dax Shepard started on the Ashton Kutcher MTV show, Punk'd. He's done various movies since, and I think he dated Jessica Simpson for a while...but who hasn't??
My first two were Matthew Broderick and Freddie Prinze. Talk about random!

John Barrowman plays Captain Jack Harkness on Torchwood, having originated the role on Doctor Who in 2005. Here's the Wikipedia entry for his character. Hmm, this is my second Doctor Who-related post this week - didn't expect that to happen.

I somehow got both Prince William and Colin Farrell on my list. I'm not sure that this makes any sense.